1. Field of the Invention
The present invention relates to the telephone art, and more particularly relates to coin-operated telephone systems which are provided with means which allow restricted coin-free service.
2. Description of the Prior Art
Standards have been developed in the telephone industry regarding certain types of toll-free calls. Such toll-free calls are generally characterized by numbers having three or less digits; for example, toll-free calls to the operator may be made by simply dialing the single digit "0". In some areas, the number "911", or a similar three digit sequence, has been designated a toll-free number for emergency fire, police and ambulance service.
Pay stations are telephone subsets from which a call can only be placed after the insertion of coinage of specific denominations. The toll-free calls referred to above may be placed from a conventional pay station, but only after the proper amount of coinage is inserted beforehand. Since many of the toll-free emergency calls described above are frequently placed from remote locations where the proper coinage is not available, or when time is of the essence, then it is desirable to provide such pay stations with means which allow restricted coin-free dialing of these emergency numbers.
There are suggestions in the prior art for such arrangements. In U.S. Pat. No. 3,678, 203, Lorange discloses a system for counting the number of dialed digits and restricting service after a preselected number of digits have been dialed. In U.S. Pat. No. 3,676,597, Peterson discloses a system which analyzes the digit value of the first three digits to determine if a proper toll-free number has been dialed. Burns, in U.S. Pat. No. 3,760,101, also teaches a system having an emergency dialing feature. These three patents are merely representative of the prior art in this area.
Some prior art coin-free pay station systems have proven to be unsatisfactory because they are susceptible to certain types of fraudulent dialing practices. One such fraudulent practice is associated with tone generator-type dialing mechanisms, and is referred to as "finger walking". This fraud constitutes the practice of sequentially dialing the digits of the called number by pressing the switch of each digit before releasing the switch of the previous digit. When such occurs, the central office recognizes a valid called number, while the coin-free circuit at the pay station "sees" only a single, continuous digit.
A second type of fraudulent practice is referred to as "hook switch dialing". This is accomplished by rapidly operating the hook switch to simulate a called number. Since most coin-free circuits reset after each hook switch operation, then a toll-free call can be placed through the central office by operating the hook switch with sufficient rapidity such that the central office recognizes a digit.
Further, many central office facilities will not complete a call from a pay station unless a coin-presence signal (referred to as "coin ground"), is received from the pay station, indicating that the proper coinage has been inserted. In U.S. patent application Ser. No. 590,003, filed on June 25, 1975, now U.S. Pat. No. 3,992,588, and which is assigned to the assignee of the present invention, I disclosed one type of a "phantom coin" circuit for use with coin-free circuits in order to satisfy this central office requirement during coin-free service.
Another problem arises with such coin-free arrangements, in that the circuit design must conform with existing pay station limitations, and be adaptable for use with a number of different pay station designs. In particular, it is often necessary to install a new coin-free circuit on working pay stations having a limited number of hook switch "leaves" available.